Natural gas flowmeter application case introduction
2021-04-29
At present, the flow meter products used for natural gas flow measurement in China are roughly: gas waist wheel flowmeter, gas turbine flowmeter, precession vortex flowmeter, vortex flowmeter, ultrasonic flowmeter and orifice flowmeter. Here is a simple comparison of the use of these flowmeters.
1. gas waist flow meter
Gas waist flowmeter, also known as Roots flowmeter, can be used not only to measure dry gas, but also to measure moisture (that is, associated gas). Because the orifice flowmeter and turbine flowmeter are not suitable for measuring the associated gas containing liquid droplets, the gas waist flowmeter has certain advantages because it has no strict requirements. In addition, another advantage is that it is not sensitive to flow pulsations.
But here are a few things to note when measuring natural gas:
a. Generally, a certain number of filters should be installed in the upstream of the flow meter to prevent the flow meter from being stuck and blocked, and to regularly discharge and check and clean the filter net.
b. In order to prevent liquid accumulation in the metering chamber of the flowmeter, the flowmeter should be installed vertically as far as possible, or higher than the process pipeline, and the liquid accumulation should be regularly discharged.
c. Before the flowmeter is put into use, it must be ensured that there are no impurities such as welding slag in the pipeline, and the bypass is opened to prevent the flow meter rotor from running at high speed and damaging the flow meter.
d. must regularly observe the pressure before and after the flow meter, timely judge the fault.
2. Gas turbine flowmeter
Gas turbine flowmeter is the most frequently used natural gas flow measuring instrument at present, with the advantages of simple structure and convenient installation; The overall size is relatively small; High accuracy; Good repeatability; The range width can reach 15∶1 ~ 25∶1, in the case of high pressure gas transmission, the range can be further increased; Its output is a pulse frequency signal, so it is easy to obtain low system uncertainty when used with a programmable flow display meter. Its shortcomings are the high-speed rotation of the turbine, the mechanical friction between the bearing and the shaft, and the life is not very long, so attention should be paid to lubrication, and the lubrication means provided by the manufacturer can be used to regularly supplement the lubricating oil. In addition, if the high-speed flow of gas contains large solid particles, it is easy to break the turbine blade, so the pipeline before the turbine flowmeter should be equipped with a filter.
3. Precession vortex flowmeter
Precession vortex flowmeter is commonly used in some small natural gas distribution stations in China. It has a wide operating temperature range. The range is large; The Reynolds number is in a certain range and is not affected by fluid temperature, pressure, density and viscosity. Strong applicability, in addition to containing larger particles or longer fiber impurities, generally do not need to install a filter; The requirements for upstream and downstream straight pipe sections are lower, and the upstream 4D and downstream 2D straight pipe sections can be used. The output frequency is linear with the volume flow rate. Its disadvantage is that the pressure loss is large, and secondly, the precession vortex flowmeter is a fluid vibration flowmeter, which is more sensitive to pipeline vibration and electromagnetic interference, so it can only be used in an environment with small vibration and no electromagnetic interference.
4. vortex flowmeter
At present, vortex flowmeter is generally used in the internal assessment of natural gas and the measurement of energy consumption in industrial and mining enterprises. It has a reasonable overall structure design, wide dynamic measurement range and small pressure loss. The main body of the instrument can be made of stainless steel, which is suitable for the measurement of corrosive media; The instrument has no moving parts, easy installation and maintenance; Field LCD display, pulse, 4 ~ 20mA output or 485 serial communication interface, can be connected with industrial automation system and many other advantages. Its biggest disadvantage is that it is characterized by fear of vibration and electromagnetic interference.
5. ultrasonic flowmeter
Because of its wide measuring range, high measurement accuracy, no pressure loss and moving parts, low installation and use costs, gas ultrasonic flowmeter has been used by hundreds of users in Europe and the United States for natural gas trade measurement. So far, the governments of 12 countries such as the United States, the Netherlands, the United Kingdom, and Germany have approved gas ultrasonic flowmeters as legal measuring instruments for natural gas. The accuracy of gas ultrasonic flowmeter is not only related to the flow rate, but also to the diameter of the instrument. For the small diameter instrument, because the length of the sound channel is short, it is difficult to measure the sound wave propagation time in turbulent gas, so the accuracy of the small diameter gas ultrasonic flowmeter is difficult to improve.
6. orifice flowmeter
For decades, people have summarized dozens of special research and practical applications for natural gas measurement, a qualitative leap has taken place on the basis of quantity, and its symbol is standardization, that is, the use of standard orifge-plate flowmeters, which can determine the relationship between signal (differential pressure) and flow without real flow calibration, and estimate its measurement error. It is the only one that meets this standard in all flowmeters at present. In order to eliminate its own output signal analog signal, repeatability is not high, narrow range, pressure loss and other major shortcomings, the use of microelectronics technology, computer technology, fixed value throttling parts and standard nozzles and other technical devices, so that its technical level has been further improved. In short, these flow meters in the measurement of natural gas flow, each has its own advantages, but also each has its own shortcomings, the user can consider their own actual situation when choosing, specific problem specific analysis, choose the right flow meter products to ensure the correct measurement.
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